The invention relates to camera tubes of the type including a photoconductive target layer comprising, primarily lead monoxide. More specifically, it relates to such camera tubes, which include internal means for regenerating the target layer to restore or reestablish desired spectral sensitivity characteristics which have deteriorated in the course of time by use, or by storage.
Camera tubes of the vidicon type having targets comprising a layer primarily of lead monoxide have been widely used for TV pickup. Such target layers have a p-i-n junction structure with the scan side of the layers exhibiting a p-type conductivity. Such camera tube targets are more fully described in an article by E. F. deHaan, A. van der Drift, and P. P. M. Schampers entitled "The Plumbicon, a new television camera tube" in Philips Technical Rev. 25, 133-151 (1963/64).
It is well known that after a number of operational hours there is a gradual deterioration of the spectral sensitivity of the camera tubes described, particularly at the blue end of the sensitivity range. The deterioration described may also occur during a period of storage.
One desired characteristic for targets used in vidicons of the type described is a relative insensitivity of its target layer to changes in the magnitude of a reverse bias voltage connected across that layer. Tubes having an excessive change in photosensitivity to such voltage changes when light within the blue region of the light spectrum is focused upon that layer may be said to display "blue slump". Generally, tubes which have displayed the deterioration previously described, also display, in operation, a blue-slump characteristic. These tubes thereafter display, by continued operation, other still more serious defects such as an inadmissibly high dark current which make them unusable. Thus, the degree of "blue slump" displayed by such tubes is used as a leading indication of device failure.
The development of poor blue operation characteristics leading to a dark current breakdown mode of failure is caused by the degradation of the hole transport properties of the target layer. This degradation may be due to loss of oxygen and/or water from the target layer. The degradation is also believed to be due to the formation of excessive hole traps in the target layer. These traps are believed to be generated by crystalline defects and/or impurity states in the surface and/or bulk produced by contaminating vapor species resulting from photoelectronic processes in the tube envelope. During operation and/or storage of such tubes, vapor species such as oxygen, hydrogen, water and hydrocarbons are transferred from the target layer to the space in the tube. These vapor species, theoretically, are absorbed by the barium which has condensed along the interior wall surface of the tube envelope as a result of the flashing of the conventional barium getter in the tube during manufacture.
The flashing of a single evaporable type barium getter during manufacture is well known in the art. Such getters are ordinarily activated subsequent to the formation of target layers and seal-off of the tubes, to clean up or absorb gases which are released from the materials of the respective tube structures. In this manner, lower pressures can be obtained within the tubes than can be obtained with pumps alone.
One approach to reestablish desired spectral sensitivity of lead monoxide target layers is described in U.S. Pat. No. 3,515,927 issued to E. F. deHaan on June 2, 1970. Disclosed therein is a method of regeneration in which a gas atmosphere having a single active constituent, a gas of the group of oxygen, water vapor, and mixtures thereof, is caused to act upon the target layer. The gas may be supplied through the exhaust tubulation or by means of a preparation incorporated within the tube capable of generating such a gas. The gas is allowed to act upon the target layer for some time and the tube is thereafter gettered. Such an approach requires exacting control to avoid chemical contamination of the target layer, cathode or other parts within the envelope. Furthermore, where gas evolving materials are incorporated within the envelope, difficulty is encountered in providing controlled heating of that material.
A simple and inexpensive approach for regenerating degenerated lead monoxide type target layers is desired, particularly for hole transport characteristics not solely degraded by excessive loss of oxygen and water.